md-option.mat-selected does not support multiple themes

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Should be able to style the selected md-option based on the currently selected theme

What is the current behavior?

Because the overlay for the select options are rendered outside of the Angular app element, theme-specific styles cannot be applied to the selected option.

What are the steps to reproduce?

With the following Sass theme setup:

@import '~@angular/material/_theming.scss';
@include mat-core();

$default-theme: mat-dark-theme(mat-palette($mat-green), mat-palette($mat-red));
@include angular-material-theme($default-theme);

.red-app {
    background-color: black;
    $red-app-primary: mat-palette($mat-red);
    $red-app-accent: mat-palette($mat-blue);
    $red-dark: mat-dark-theme($red-app-primary, $red-app-accent);
    @include angular-material-theme($red-dark);
}

.red-app .md-option.mat-selected.map-primary,
.red-app .mat-primary .mat-option.mat-selected {
    color: map-get($mat-red, 400);
}

The styles relative to .mat-selected will never be set because it is never rendered inside of an element containing the .red-app theme class:

For sample implementation, see http://material-platform.azurewebsites.net/select

What is the use-case or motivation for changing an existing behavior?

md-option should be able to be styled by the currently active theme.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Given the nature of the way the overlay for md-option has been implemented, it would affect any version of Angular and TypeScript, as well as any OS and browser.

Material: 2.0.0-beta.7

[willshowell]

@JaimeStill overlay components require a little extra setup for multiple styles

https://github.com/angular/material2/blob/master/guides/theming.md#multiple-themes-and-overlay-based-components

Did you update the themeClass?

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